According to a previously known method, the big-end is divided into an upper bearing and a bearing cap, which are reconnected by means of bolts, when the complete engine is assembled. The division can be effected in different ways, e.g. by milling apart which is, however, both labour and material consuming, since the connecting rod must be overdimensioned due to the material which is milled off. This in turn requires that the crankshaft hole in the big-end first be drilled in circular shape and then be worked to an oval and that the milling off of material during division be so exact that the crankshaft hole, when the parts are re-assembled, will again assume a circular shape fitting the crankshaft. In order to make possible re-assembly of the bearing cap and the upper bearing with exact fit, the separate parts must also be specially worked. The working is, as a rule, carried out by deep grinding or serration grinding and cutting grooves, i.e. alternating ridges and grooves are made in the facing surfaces of the upper bearing and the bearing cap to provide the required guidance for correct assembly of the parts.
Another known process means that the bearing cap is separated from the big-end of the connecting rod by buckling. In order to avoid deformation of the material adjacent the dividing plane, it must, as a rule, be pretreated prior to the buckling blow itself. The pretreatment is intended to alter the properties of the material surrounding the plane of division, to increase its brittleness, which is advantageous to the buckling step. One method of treatment involves an electrolytic process, by means of which hydrogen is caused to diffuse into the metal along the intended plane of division, whereafter a sufficiently heavy blow or other force produces a fracture without deformation. Another method involves increasing the brittleness of the material in the area in question by means of induction hardening. To be sure, by means of this process, the buckling plane through the remaining material in the upper bearing and the bearing cap will form after buckling opposing irregularly profiled surfaces which have varying profile depths depending on the hardness of the material. The projections and indentations in the profile surfaces guide the upper bearing and the bearing cap into exact alignment with each other.
However, this division according to known methods is complicated and requires special tools for execution, and therefore it is desirable that at least some treatment or working step can be simplified or eliminated. The operations in question include draw reaming of bearing caps, separation by sawing, serration grinding, disassembly, washing and re-assembly of each connecting rod and finishing of the crankshaft hole and buckling in a special buckling machine. A reduction in the number of operations can also make it possible to reduce the number of operators.
The purpose of the invention is therefore to improve known methods of manufacturing connecting rods by a method which is simple, inexpensive and can be carried out with high precision.